Atrial natriuretic peptide (ANP) is found to be elevated in preeclamptic patients despite the presence of hemodynamic characteristics such as vasoconstriction and hypovolemia. In this study, the effect on ANP secretion of plasma volume expansion with crystalloid solutions was investigated. Seven preeclamptic and seven normotensive pregnant women in their last trimester were compared. After basal ANP measurements, 0.9% Ringer's solution, 15 cm3/kg body weight was infused within 30 min to expand the plasma volume. Blood sampling was repeated after the infusion. Maternal blood volume expansion was calculated from the decrease in hematocrit. Plasma ANP levels were corrected according to the degree of volume expansion. Basal mean levels in both groups were not different. Although the postinfusion levels of ANP in normotensive pregnant women were similar to the basal levels (means +/- SE: 20.6 +/- 0.41 and 27.2 +/- 0.52 pg/ml, respectively, p = 0.10), the postinfusion ANP levels in preeclamptic women increased significantly (21.4 +/- 0.31 and 34.1 +/- 0.28 pg/ml, respectively, p = 0.01). Preeclampsia is associated with a greater increase in plasma ANP levels in response to volume expansion compared with normotensive pregnancy. The greater change in the right atrial pressure with volume expansion, due to decreased compliance of the capacitance vessels in preeclamptic subjects might explain the greater change in the plasma level of ANP.